Hey Jim. We call it a spike jig for want of a better name. We use it to mark out our dowel positions. Just a straight stick with a stop at one end, screws set at dowel positions that when pressed on the timbers surface, leaves little pin holes. Holes then drilled on horizontal borer.

-- Andy Halewoodworker. You can't finish if you don't finish. So finish it, because finish is everything.